This invention relates to planar type thickness shear mode quartz resonators or vibrators or oscillators such as AT-cut quartz resonators and more particularly to quartz resonators designed to oscillate in the third overtone.
Conventionally, in planar type quartz resonators or oscillators among thickness shear mode quartz oscillators, usually the proportions of diameters (D) and thicknesses (T) are so designed to oscillate the oscillators in fundamental tones. Commonly the proportion is within the range from D/T=50 to D/T=100. The reason is as follows. Oscillation of a quartz oscillator is determined by the difference between the negative resistance of an oscillation circuit and the crystal impedance of the quartz oscillator, and the quartz oscillator oscillates in an oscillation mode in which the difference is greatest. The negative resistance of an oscillation circuit tends to decrease in proportion as the frequency becomes high. In other words, the negative resistance decreases in proportion as the ratio D/T increases. Whereas the impedance of a quartz oscillator shows a sharp decline curve with respect to the proportion D/T, and impedance in fundamental tone becomes lowest among impedances in all oscillation modes near or above D/T=45. The characteristic curve of negative resistance of an oscillation circuit varies according to the IC (integrated circuit) used in the oscillation circuit, and the impedance of a quartz oscillator changes with the diameter and other factors. The aforementioned relation, however, exists fundamentally and the proportion D/T is usually designed within the range from 50 to 100 so as to get stable oscillation in fundamental tone. Some conventional planar type quartz oscillators are driven in third overtone, but in such case, oscillation circuits are provided with tuning circuits composed of coils (L) and condensers (C).
In the prior art, as described above, very thin quartz plates of D/T=50.about.100 are employed and such quartz oscillators are subjected to the influence of supporting means, and it has been difficult to realize high, stable oscillations due to the decrease in the quality factor (Q). Further, the thickness of the quartz oscillators is so thin that they are low in shock resistance and apt to break. Further, in an oscillation device which is provided with a tuning circuit with L (coils) and C (condensers) for getting third overtone, it is inevitable that the device must be large due to the tuning circuit, especially the coil (L), and the structure becomes complex with the increased number of parts.
The present invention provides a planar type thickness shear mode quartz oscillator to be operated by an oscillation circuit without a L, C (coils, condensers) tuning circuit in which the ratio between the diameter and the thickness of the oscillator is so designed as to perform stable oscillation in third overtone.